1. Field of the Invention
The present invention relates to a video signal processing apparatus and method, and more particularly, to a digital broadcast signal processing apparatus and method in which channel interference by an analog broadcast is prevented, for effectively removing an analog broadcast signal mixed in a digital broadcast signal of a co-channel.
2. Description of the Related Art
Broadcast stations have developed a digital broadcast system, with improved picture quality and sound quality compared to an analog broadcast system. However, it has been decided that, until the use of digital televisions become more prevalent, a conventional analog broadcast would be transmitted together with a digital broadcast.
The FCC of the United States decided earlier to change the broadcasting methods. According to the earlier decision, analog television broadcast method (the NTSC broadcast standard) and the digital television broadcast method will be simultaneously transmitted until 2006. After that, only digital television broadcast will be transmitted and analog television broadcast will be terminated.
However, later the FCC changed the earlier decision to simultaneously broadcast the two signals until digital television broadcast receivers are used in 85% of households.
In the advanced television services committee (ATSC) standard which is currently the standard digital television method in the United States, an NTSC rejection filter (NRF) block 1000 is added to a channel decoder of the digital television as shown in FIG. 1. The filter is added to prevent the deterioration of receiving characteristics generated in the digital television receiver by co-channel interference of an analog television channel and a digital television channel. Such an interference can be generated when analog and digital television signals are simultaneously broadcast. By such a filtering, the analog television broadcast signal (NTSC) component mixed in the digital television channel is removed, and thus does not generate channel interference.
As shown in FIG. 1, a conventional digital broadcast signal processing apparatus includes an antenna 101, a tuner 102, a saw filter 103, an intermediate frequency (IF) detector 104, an analog-to-digital (A/D) converter 105, a controller 106, a carrier recovery unit 107, a multiplier 108, a matched filter 109, an NTSC rejection filter (NRF) 110, an NTSC interference discriminator 111, and a multiplexer 112.
A block including the tuner 102, the saw filter 103, and the IF detector 104 is called a tuning block.
A block including the carrier recovery unit 107, the multiplier 108, the matched filter 109, the NRF 110, the NTSC interference discriminator 111, and the multiplexer 112 is called a channel-decoding block 1000.
However, the ATSC standard is designed such that the NRF is effective only when there is little RF frequency offset of each channel transmitted by the broadcast stations.
In actual practice, the correctness of an RF broadcast frequency deteriorates since the offset amount of a transmitter is large and the offset amount of a transmitted RF frequency becomes significantly large while a signal passes through a cable and a repeater.
According to the conventional technology, when the offset amount of the transmitted RF frequency is large, it is not possible to effectively attenuate the analog broadcast signal due to the sharp frequency characteristic of a comb filter which constitutes the NRF. Accordingly, co-channel interference occurs when the broadcast signal of the digital television is received, thus deteriorating reception sensitivity.
To solve the above problem, it is an object of the present invention to provide a digital broadcast signal processing apparatus in which channel interference by an analog broadcast is prevented, for effectively preventing a channel interference phenomenon by automatically correcting the amount of change of an RF broadcast frequency and applying the amount of change to an NTSC rejection filter (NRF) when co-channel interference is generated by an analog broadcast signal when a digital television broadcast signal is received.
It is another object of the present invention to provide a digital broadcast signal processing method in which channel interference by an analog broadcast is prevented, for effectively preventing a channel interference phenomenon by automatically correcting the amount of change of an RF broadcast frequency and applying the amount of change to an NTSC rejection filter (NRF) when co-channel interference is generated by an analog broadcast signal when a digital television broadcast signal is received.
Accordingly, to achieve the first object, A signal processing apparatus for removing an interfering analog broadcast signal from a digital broadcast signal, said digital broadcast signal being received from a tuning block, comprising a carrier frequency detector for detecting a frequency of a carrier signal recovered by a carrier recovery unit, an offset frequency operating unit to generate a difference xcex94f between the carrier frequency f0 detected by the carrier frequency detector and a carrier set frequency fc set by a standard specification and output the difference xcex94f and a controller to generate a -tuning control signal for changing a local oscillation frequency of the tuning block corresponding to the magnitude of the difference xcex94f.
Preferably the carrier recovery unit recovers a carrier signal obtained by removing a modulation signal from the digital broadcast signal, and the apparatus further comprises a multiplier to perform demodulation by multiplying the digital broadcast signal with the carrier signal recovered by the carrier recovery unit, an NTSC rejection filter (NRF) to remove the analog broadcast signal component included in the output signal component demodulated by the multiplier, a multiplexer to receive the output signal of the NRF and the output signal of a matched filter, and to select a received signal in accordance with a selection control signal, and to output the selected signal and an NTSC interference discriminator to receive the output signal of the matched filter and the output signal of the NRF, to generate the signal-to-noise ratios of the two received signals, and to generate the selection control signal.
Still preferably the control signal enables the selection of the output signal of the NRF when the difference between the signal-to-noise ratio of the output signal of the matched filter and the signal-to-noise ratio of the output signal of the NRF is larger than a set value and said control signal enables the selection of the output signal of the matched filter when the difference between the signal-to-noise ratio of the output signal of the matched filter and the signal-to-noise ratio of the output signal of the NRF is smaller than or equal to the set value.
To achieve the second object, there is provided a signal processing method for removing an interfering analog broadcast signal from a digital broadcast signal, which is received from a tuning block, comprising a) setting a standard specification frequency fc of a carrier signal and a threshold offset frequency xcex94fc, (b) detecting the frequency f0 of a carrier signal by detecting the carrier signal from the digital broadcast signal, (c) generating a difference xcex94f between the standard specification frequency fc and the frequency fo of the carrier signal, (d) comparing an absolute value of the difference xcex94f and the threshold offset frequency xcex94fc and feeding back to the step (b) when the absolute value of the difference xcex94f is smaller than the threshold offset frequency xcex94fc, (e) correcting the local oscillation tuning frequency of the tuning block in proportion to the magnitude of the difference xcex94f when the absolute value of the difference xcex94f is larger than or equal to the threshold offset frequency xcex94fc, (f) determining whether an interference of the analog broadcast signal is generated in a process of demodulating the digital broadcast signal and (g) outputting a signal passed through an NTSC rejection filter (NRF) when it is determined that the interference is generated in the step (f) and outputting a signal which did not pass through the NRF when it is determined that the interference is not generated in the step (f).